In the papermaking art, an aqueous suspension containing cellulosic fibres and optional fillers and additives, referred to as stock, is fed into a headbox which ejects the stock onto a forming wire. Water is drained from the stock so that a wet web of paper is formed on the wire, and the web is further dewatered and dried in the drying section of the paper machine. Drainage and retention aids are conventionally introduced into the stock in order to facilitate drainage and to increase adsorption of fine particles onto the cellulosic fibres so that they are retained with the fibres on the wire.
Sols of silica-based particles are widely used as drainage and retention aids in combination with charged organic polymers. Such additive systems are among the most efficient now in use in the papermaking industry. One of the parameters affecting the properties and performance of silica-based sols is the specific surface area; stable, high-performance silica-based sols usually contain particles with a specific surface area of at least 300 m2/g. Another parameter is the S value, which indicates the degree of aggregate or microgel formation; a lower S-value is indicative of a higher degree of aggregation. While high surface areas and a certain degree of aggregate or microgel formation may be advantageous from a performance point of view, very high surface areas and extensive particle aggregation or microgel formation result in considerably decreased stability of silica-based sols, thereby making extreme dilution of the sols necessary so as to avoid gel formation.
U.S. Pat. No. 5,368,833 discloses a silica sol comprising silica particles having a specific surface area within the range of from 750 to 1,000 m2/g which are surface-modified with aluminium to a degree of from 2 to 25% substitution of silicon atoms, and wherein the sol has an S value within the range of from 8 to 45%. Said patent also discloses a process for producing the silica sol which comprises the steps of acidifying a water glass solution to a pH within the range of from 1 to 4; alkalising the acid sol at an SiO2 content within the range of from 7 to 4.5% by weight; allowing particle growth of the sol to a specific surface area within the range of from 750 to 1,000 m2/g; and subjecting the sol to aluminium modification.
U.S. Pat. No. 5,603,805 discloses silica sols having an S value within the range of from 15 to 40% comprising anionic silica particles, said silica particles optionally being aluminium modified, and having a specific surface area within the range of from 300 to 700 m2/g. Said patent also discloses a process for producing the silica sol comprising the steps of acidifying a water glass solution to a pH within the range of from 1 to 4; alkalising the acid sol at an SiO2 content within the range of from 7 to 5% by weight; alternatively alkalisation of the acid sol to a pH value between 7 and 9; and particle growth of the sol to a specific surface area within the range of from 300 to 700 m2/g; and optionally followed by aluminium modification.
International Patent Appln. Publ. No. WO 98/56715 discloses a process for preparing an aqueous polysilicate microgel which comprises mixing an aqueous solution of an alkali metal silicate with an aqueous phase of a silica-based material having a pH of 11 or less. The polysilicate microgel is used as a flocculating agent in combination with at least one cationic or amphoteric polymer in the production of pulp and paper and for water purification.
International Patent Appln. Publ. No. WO 00/66492 discloses a process for the production of an aqueous sol containing silica-based particles which comprises acidifying an aqueous silicate solution to a pH of from 1 to 4 to form an acid sol; alkalising the acid sol in a first alkalisation step; allowing particle growth of the alkalised sol for at least 10 minutes and/or heat-treating the alkalised sol at a temperature of at least 30° C.; alkalising the obtained sol in a second alkalisation step; and optionally modifying the silica-based sol with, for example, aluminium.
U.S. Pat. No. 6,372,806 discloses a process for preparing a stable colloidal silica having an S-value of from 20-50 and wherein said silica has a surface area of greater than 700 m2/g comprising: (a) charging a reaction vessel with a cationic ion exchange resin having at least 40 percent of its ion exchange capacity in the hydrogen form wherein said reaction vessel has means for separating said colloidal silica from said ion exchange resin; (b) charging said reaction vessel with an aqueous alkali metal silicate having a mole ratio of SiO2 to alkali metal oxide in the range of from 15:1 to 1:1 and a pH of at least 10.0; (c) stirring the contents of said reaction vessel until the pH of said contents is in the range of from 8.5 to 11.0; (d) adjusting the pH of the contents of said reaction vessel to above 10.0 using an additional amount of said alkali metal silicate; and (e) separating the resulting colloidal silica from said ion exchange resin while removing said colloidal silica from said reaction vessel.
U.S. Pat. No. 5,176,891 discloses a method for the production of water soluble polyaluminosilicate microgels having a surface area of at least about 1,000 m2/g, comprising the steps of (a) acidifying a dilute solution of alkali metal silicate containing about 0.1 to 6 wt. % SiO2 to a pH of between 2 and 10.5 to produce polysilicic acid; followed by (b) reacting a water soluble aluminate with the polysilicic acid before the polysilicic acid has gelled such that a product with an alumina/silica mole ratio greater than about 1/100 is obtained; and then (c) diluting the reaction mix before gelation has occurred to the equivalence of about 2.0 wt. % SiO2 or less to stabilize the microgels.
It would be advantageous to be able to provide silica-based sols with high stability and SiO2 contents as well as improved drainage performance. It would also be advantageous to be able to provide improved processes for the preparation of silica-based sols with stability and SiO2 contents as well as improved drainage performance. It would also be advantageous to be able to provide a papermaking process with improved drainage.